The invention relates to a semiconductor device and more particularly, to a wiring structure which connects conductive layers with tungsten.
A typical known wiring structure of a semiconductor device in which two conductive layers are electrically connected by alpha tungsten (hereafter ".alpha.-W") extending through a contact hole in an insulating layer between the conductive layers is illustrated in FIG. 1. Referring to FIG. 1, such a wiring structure can be formed as follows: A field oxide layer 12 is formed on a field region of a silicon substrate 11 by a selective oxidation, i.e., the well known local oxidation of silicon method (hereafter "LOCOS method"). Then a diffusion region 13 is formed in an active region of the substrate 11. An insulating layer 14, such as layer of boron phosphorus silicate glass (hereafter "BPSG"), is deposited over the entire surface of the substrate 11 by the chemical vapor deposition (hereafter "CVD") method. The insulating layer 14 is subjected to a heat treatment ot flatten the surface thereof. Subsequently, the insulating layer 14 is selectively removed by photolithographic etching to form a contact hole 15. An .alpha.-W layer 16 is selectively deposited only in the contact hole 15 by a selective CVD method. The method includes application to the hole of a gaseous mixture of SiH.sub.4 and WF.sub.6 in a ratio SiH.sub.4 /WF.sub.6 &lt;1.0 at a gas pressure of 0.01-0.3 Torr and temperature of 230.degree.-400 .degree. C. A wiring layer 17 formed of Al-Si alloy is deposited on the insulating layer 14 and the .alpha.-W layer 16 by a sputtering method. The .alpha.-W layer 16 provides an electrical connection between the diffusion layer 13 (one conductive layer) and the Al-Si alloy layer 17 (another conductive layer). An electrical connection provided by the above method prevents disconnection between conductive layers and provides a highly reliable wiring structure.
However, during a further heat treatment performed after the formation of the Al-Si alloy layer, the .alpha.-W 16 reacts with Al-Si alloy and the .alpha.-W is diffused into the Al-Si alloy layer 17 to form an .alpha.-W diffused Al-Si alloy region 18 in the Al-Si alloy layer 17, as shown in FIG. 2. The higher the temperature at which the heat treatment is performed, the more .alpha.-W reacts with the Al-Si alloy. Since the resistance of the .alpha.-W diffused Al-Si alloy region 18 is higher than that of the Al-Si alloy layer 17, the resistance of the wiring layer increases in because of the heat treatment.